A ridge waveguide typically comprises material remaining from an original uniform multi-layered planar waveguide whose layers are designed to confine light in-plane. The ridge is typically created by means of a selective etching process used to excavate material from either side of the ridge. In common parlance, a strong waveguide results from excavation at least deep enough to completely expose the optical guided-mode to the walls of the ridge. A weak waveguide results from a less-deep excavation in which a high refractive index ‘core’ layer (which confines the optical mode vertically) is not removed. Typically, for a weak waveguide the excavation is stopped at or above the upper boundary of this core layer, and for a strong waveguide the excavation is stopped beneath the lower boundary of this core layer, usually such that the level of the peak intensity of an optical mode guided by the waveguide falls within the ridge. However, the strength of lateral confinement of a ridge waveguide may be graded continuously from zero through weak to strong as a function of the etch depth. There are differing advantages and disadvantages to each type, and it may be desired to use both for different purposes within a single optical guided-wave device.
An optical mode that is transmitted along a composite waveguide in which a weak waveguide and a strong waveguide are butt-coupled is partially reflected from the interface between the two types of waveguide, producing back-scattering.
International patent application WO 02/31587 discloses integrated semiconductor optical devices, in which a ridge waveguide includes a guided wave spatial filter for filtering out optical radiation modes that are higher-order than the fundamental optical radiation mode. The guided wave spatial filter comprises a section of the ridge waveguide having relatively deeply etched regions on each side of the ridge of the waveguide, in comparison to main sections of the ridge waveguide in which there are relatively shallowly etched regions on each side of the ridge of the waveguide. In two disclosed versions, the devices include a mixing region fabricated in a relatively deeply etched region.